Sample return mission

A sample return mission is a spacecraft mission with the goal of collecting and returning with tangible samples from an extraterrestrial location to Earth for analysis. Sample return missions may bring back merely atoms and molecules or a deposit of complex compounds such as loose material ("soil") and rocks. These samples may be obtained in a number of ways, including a collector array used for capturing particles of solar wind or cometary debris, soil and rock excavation, mining, and any other possible way for retrieving samples in the environment.

Up to the present, humanity has collected samples of six identified Solar System bodies as well as samples of the solar wind. These samples were acquired through three methods: The collection of samples of Earth itself, the collection of meteoroids that have fallen on Earth, and the collection of samples through sample return missions. Samples of Moon rock from Earth's Moon were collected both from meteorites and through unmanned and manned sample return missions. The comet Wild 2 and the asteroid 25143 Itokawa were visited by unmanned spacecraft which returned samples to Earth. Furthermore samples for three identified Solar System bodies were only collected by means other than sample return missions: These are samples from Earth itself, samples from Vesta in the form of HED meteorites and samples from Mars in the form of Martian meteorites.

Samples available on Earth can be analyzed in laboratories so we can further our understanding and knowledge as part of the discovery and exploration of the Solar System. Until now many important scientific discoveries about the Solar System were made remotely with telescopes, and some Solar System bodies were visited by orbiting or even landing spacecraft with instruments capable of remote sensing or sample analysis. While such an investigation of our Solar System is technically easier than a sample return mission, the scientific tools available here on Earth to study such samples are far more advanced and diverse than those that can go on spacecraft. Analysis of samples on Earth allows to follow up any findings with different tools, including tools that have yet to be developed – in contrast a spacecraft can carry only a limited set of analytic tools and these have to be chosen and built long before the spacecraft reaches its target.

Samples analyzed on Earth can be matched against findings of remote sensing, for more insight into the processes that formed the Solar System. This was done for example with findings by the Dawn spacecraft which visited the asteroid Vesta from 2011 to 2012 for imaging, and samples from HED meteorites (collected on Earth until then) which were compared to data gathered by Dawn.[1] These meteorites could then be identified as material ejected from the large impact crater Rheasilvia on Vesta. This allowed deducing the composition of crust, mantle and core of Vesta. Similarly some differences in composition of asteroids (and to a lesser extent different compositions of comets) can be discerned by imaging alone. However to get a more precise inventory of the material present on these different bodies more samples will be collected and returned in the future, to match their compositions with the data gathered through telescopes.

One further focus of such investigation – besides the basic composition and geologic history of the various Solar System bodies – is the presence of the building blocks of life on comets, asteroids, Mars or the moons of the gas giants. Several sample return missions to asteroids and comets are currently in the works. More samples from asteroids and comets will help determine whether life formed in space and was carried to Earth in the form of meteorites. Another question under investigation is whether extraterrestrial life formed on other Solar System bodies like Mars or on the moons of the gas giants, and if life might even exist there today. The result of NASA's last "Decadal Survey" was to prioritize a Mars sample return mission, as Mars has a special importance: it is comparatively "nearby", might have harbored life in the past, and might even be able to sustain life today. Jupiter's moon Europa is another important focus in the search for life in our Solar System. However due to the distance and other constraints Europa might not be the target of a sample return mission in the foreseeable future.

A sample return from Mars or Europa or other location with possibility to harbour life is a Category V mission under COSPAR. No sample has yet been returned with alien life in it, so we have no past experience to guide us. In the most interesting case then a sample returned from these locations might contain such life. It's not known what the effects of that are likely to be on humans or the environment of the Earth.

It might be that it would have no effect due to the dominance of present life on Earth and because it would not be adapted to infect human or DNA based life. You can also argue the other way however, that in worst case, that we would not be adapted to be able to resist it, and that alien life would not be adapted to have no harmful effects (pathogens are most lethal when they first migrate to a new host).[2] Also some diseases can leap directly to an animal host, such as Legionnaire's disease, jumped straight to humans, previously a disease of amoeba.

For these reasons, Carl Sagan and Joshua Lederberg both argued that we should only do sample return missions classified as Category V missions with extreme caution, and this has also been confirmed by later studies by the NRC and ESF.[2][3][4][5][6]

After two failed Soviet attempts to return lunar soil to Earth through unmanned robotic Luna missions in June and July 1969,[7] the manned US Apollo 11 mission in July 1969 achieved the first successful sample return from another Solar System body. It returned approximately 22 kilograms (49 lb) of Lunar surface material. This was followed by 34 kilograms (75 lb) of material from Apollo 12 and further 326 kilograms (719 lb) material from four more missions from the manned Apollo program.

Perhaps one of the most significant advances in sample return missions occurred in 1970 when the robotic Soviet mission known as Luna 16, successfully returned 101 grams (3.6 oz) of lunar soil. Likewise, Luna 20 returned 55 grams (1.9 oz) in 1974 and Luna 24 returned 170 grams (6.0 oz) in 1976. Although they recovered far less than the Apollo missions, they did this fully automatically.

In 1970, the Soviet Union planned for a 1975 first Martian sample return mission in the Mars 5NM project. This mission was planned to use an N1 superrocket, but as this rocket never flew successfully, the mission got updated to use a double launch with the smaller Proton rocket, and an assembly at a Salyut space station. This Mars 79 mission was planned for 1979, but got cancelled in 1977 and all hardware was ordered destroyed.[8]

After the last sample return mission by Luna 24 in 1976, twenty years passed before the Earth-Orbital Debris Collection (ODC) experiment collected extraterrestrial samples. The Experiment was deployed on the Mir space station for 18 months during 1996–1997 and used aerogel to capture particles from low-Earth orbit, consisting of interplanetary dust and man-made particles.

An artist's rendering of Genesis while collecting solar wind

The next mission to return extraterrestrial samples was known as Genesis – it was able to return solar wind samples to Earth from beyond Earth orbit. Unfortunately, the Genesis capsule failed to open its parachute while re-entering the Earth's atmosphere, and it crash-landed in the Utah desert in 2004. There were fears of severe contamination or even total mission loss, but scientists have managed to save quite a bit of the samples—which were the first to be collected from beyond lunar orbit. Genesis used a collector array made of wafers of ultra-pure silicon, gold, sapphire, and diamond. Each different wafer was used to collect a different part of the solar wind.

Sample return capsule from the Stardust mission

Genesis was followed by NASA's Stardust spacecraft which returned comet samples to Earth January 15, 2006. It safely passed by Comet Wild 2 and collected dust samples from the comet's coma while imaging the comet's nucleus. Stardust used a collector array made of low-density aerogel (99% of which is empty space) which has about 1/1000 of the density of glass. This permits the ability to collect the cometary particles without damaging them due to high impact velocities. Particle collisions with even slightly porous solid collectors would result in destruction of those particles and damage to the collection apparatus.

The Russian Fobos-Grunt was a failed sample return mission that was supposed to return samples from Phobos, one of the moons of Mars. It was launched on November 8, 2011. However the probe failed to leave Earth orbit and crashed after some weeks into the southern Pacific Ocean.[10][11]

Illustration of OSIRIS-REx collecting a sample from asteroid 1999 RQ36

The Japan Aerospace Exploration Agency (JAXA) plans to launch the improved Hayabusa 2 space probe on November 30, 2014 and to return asteroid samples by 2020. Current target for the mission is the C-type asteroid(162173) 1999 JU3.

The OSIRIS-REx mission is scheduled to be launched in 2016 on a mission to return samples of from asteroid101955 Bennu. The samples are expected to enable scientists to learn more about the time before the birth of our Solar System, initial stages of planet formation, and the source of organic compounds which led to the formation of life.[12]

An artist's impression of a Mars sample return mission launching its payload back to Earth

There were plans to launch a Mars Sample Return (MSR) mission in 2004, but following the twin-failures of the Mars Climate Orbiter and Mars Polar Lander, MSR was cancelled. NASA has long planned a Martian sample return mission, but has yet to secure the budget to successfully design, build, launch, and land a probe that would do just that. There have been mission proposals in the past, but most have not made it far beyond the drawing boards. The mission remained on NASA's roadmap for planetary science as of the 2013 Planetary Science Decadal Survey.[13] A Mars sample return mission in collaboration with Europe (as part of the Aurora programme) was proposed launch around 2018. Due to budget cuts at NASA the future of this mission is uncertain.

Furthermore Russia has plans for Luna-Grunt mission to return samples from the Moon by 2021 and Mars-Grunt to return samples from Mars five to ten years later.

In addition OpenLuna is planning a first open source Boomerang-class lunar sample return mission.

In September 2012, NASA announced plans to further study several strategies of returning a sample of Mars to Earth - including a multiple launch scenario, a single launch scenario and a multiple rovers scenario - for a mission beginning as early as 2018.[14]

A collector array may be used to collect millions or billions of atoms, molecules, and fine particulates by using a number of wafers made of different elements. The molecular structure of these wafers allows for the collection of various sizes of particles. Collector arrays, such as those flown on Genesis are ultra-pure in order to ensure maximum collection efficiency, durability, and analytical distinguishability.

Collector arrays are useful for collecting tiny, fast-moving atoms such as those expelled by the Sun through solar wind, but can also be used for collection of larger particles such as those found in the coma of a comet. The NASA spacecraft known as Stardust implemented this technique. However, due to the high speeds and size of the particles that make up the coma and the area nearby, a dense solid-state collector array was not viable. As a result, another means for collecting samples had to be designed as to preserve the safety of the spacecraft and the samples themselves.

Aerogel is a silicon-based, porous, solid with a sponge-like structure in which 99.8% of its volume is composed of empty space. Aerogel has about 1/1000 of the density of glass. An aerogel was implemented for use with the Stardust spacecraft because the collision of a particle would have an impact velocity of about six times the speed of a rifle bullet, and hence a collision with a dense solid could alter its chemical composition, and perhaps vaporize it completely.

Since the aerogel is mostly transparent and the particles leave a carrot-shaped path once they penetrate the surface, scientists can easily find and retrieve them. Since its pores are on the nanometer scale, particles, even ones smaller than a grain of sand, do not merely pass through the aerogel completely. Instead, they slow to a stop and then are embedded within it.

The Stardust spacecraft has a tennis racket shaped collector with aerogel fitted to it. The collector is retracted into its capsule for safe-storage and delivery back to Earth. One thing that makes aerogel a good choice for missions such as Stardust is that it is quite strong and easily survives both launching and outer space environments.

Some of the most risky and difficult types of sample return missions are those that require landing on an extraterrestrial body such as an asteroid, moon, or planet. It takes a great deal of time, money, and technical ability in order to even initiate such plans. It is a difficult feat that requires that everything from launch to landing to retrieval and launch back to Earth be planned out with high precision and accuracy.

This type of sample return, although having the most risks, is the most rewarding for planetary science. Furthermore, such missions carry a great deal of public outreach potential, which is an important attribute for space exploration when it comes to publicity.

NASA is considering launching an international sample return mission of this type to Mars around the year 2018, depending on its budget. Previous attempts to launch this type of sample return mission have been scrubbed due to technical difficulty, budget constraints, and other factors such as recent mission failures (e.g.: Mars Climate Orbiter and Mars Polar Lander). The only successful robotic sample return missions of this type have been the Soviet Luna landers.